1. Field of the Invention
The present invention relates to a transparent protective film for polarizing plate and a process for producing the same. Further, the present invention relates to a polarizing plate using the transparent protective film for polarizing plate. The polarizing plate of the invention can be used independently or as an optical film laminating the plate and form a visual display, such as a liquid crystal display, an organic EL display or PDP.
2. Description of the Related Art
In a liquid crystal display, it is indispensable that polarizers should be arranged at both sides of a glass substrate that forms a top surface of a liquid crystal panel according to a picture formation method. Generally, polarizing plate laminating a polarizer, comprising a polyvinyl alcohol derived film and a dichroism substance such as iodine, and a transparent protective film such as triacetyl cellulose film is used.
The adhesion between the triacetyl cellulose film used as the transparent protective film and the polyvinyl alcohol derived film used as the polarizer is not good. Therefore, in order to improve its adhesion the surface of the triacetyl cellulose film used is saponified by dipping the film in an alkali solution. However, since the alkali solution at high concentration is used, the saponification can be dangerous and easily causes troubles such as breakdown on the facilities with large word load. Further, when the saponification is conducted for a long time, the concentration of the alkali solution is reduced and the effect of the saponification on improvement of adhesion may be inadequate. There is a further problem that disposal of the waste alkali solution generates a large amount of wastewater.
In place of the saponification of the surface of the transparent protective film, the film may be subjected to irradiation with UV rays from a low-pressure mercury lamp etc., whereby the surface thereof can be rendered hydrophilic to facilitate adhesion thereof to a polarizer. However, when such irradiation with UV rays is conducted at low power output, the treatment of facilitating adhesion cannot be effected in a short time, and a long time is required until the treatment of facilitating adhesion is completed. On the other hand, when the film is subjected to irradiation with UV rays at high power output for reducing the irradiation time, the film may be deformed by heating, or a plasticizer contained in the film is segregated on the surface by heating to prevent the surface being rendered hydrophilic, thus failing to make the surface sufficiently adherent.